Method and apparatus for adjacent service area handoff in communication systems

ABSTRACT

A handoff technique in which system users detect transitions in service between a current service area and an adjacent service area, and request a forward link channel in the new service area when a detected signal strength for the new service area exceeds predetermined threshold levels. The forward communications link in the current service area is maintained until the strength of the new service area signal reaches a certain level and appropriate channel quality is confirmed, as based on various known criteria. Typically, service area transitions are detected using the signal strength of pilot or paging signals associated with service areas, which are used to determine a relative signal strength of new service area signals. Pilot signal level adjustments used to counter roll-off effects are detected and compensated for in comparing signal levels. To minimize the burden on system resources, new service area signals are not selectable until a minimum time has passed, or a minimum change in energy from a prior pilot signal is detected. In addition, communications using the previous service area can be dropped almost immediately upon engaging the new service area.

BACKGROUND OF THE INVENTION

[0001] I. Field of the invention

[0002] The present invention relates to performing signal handoff incommunication systems, such as wireless data or telephone systems, usingsatellites. More particularly, the invention relates to a method andapparatus for handing off user terminal communication links betweendifferent satellite beams associated with a single communicationssatellite, or sectors in a single cell.

[0003] II. Description of the Related Art

[0004] A variety of multiple access communication systems and techniqueshave been developed for transferring information among a large number ofsystem users, such as code division multiple access (CDMA) spreadspectrum techniques. CDMA techniques in multiple access communicationsystems are disclosed in the teachings of U.S. Pat. No. 4,901,307, whichissued Feb. 13, 1990 under the title “Spread Spectrum Multiple AccessCommunication System Using Satellite Or Terrestrial Repeaters”, and U.S.patent application Ser. No. 08/368,570, filed under the title “MethodAnd Apparatus For Using Full Spectrum Transmitted Power In A SpreadSpectrum Communication System For Tracking Individual Recipient PhaseTime And Energy,” which are both assigned to the assignee of the presentinvention, and incorporated herein by reference. These patents disclosecommunication systems in which communication signals are transferredthrough satellite repeaters and gateways, or terrestrial base stations(also referred to as cell-sites or cells).

[0005] In a typical spread-spectrum communication system, one or morepreselected pseudorandom noise (PN) code sequences are used to modulateor “spread” user information signals over a predetermined spectral bandprior to modulation onto a carrier signal for transmission ascommunication signals. PN spreading is a method of spread-spectrumtransmission that is well known in the art, and produces a communicationsignal with a bandwidth much greater than that of the data signal. Inthe base station- or gateway-to-user communication link, PN spreadingcodes or binary sequences are used to discriminate between signalstransmitted by different base stations or over different beams, as wellas between multipath signals. These codes are typically shared by allcommunication signals within a given cell or beam, that are on a commonfrequency (sub-beam).

[0006] In a typical CDMA spread-spectrum communication system,channelizing codes are used to discriminate between different userswithin a cell or between user signals transmitted within a satellitesub-beam on a forward link (i.e., the signal path from the base stationor gateway to the user transceiver). That is, each user transceiver hasits own orthogonal channel provided on the forward link by using aunique ‘channelizing’ orthogonal code. Walsh functions are generallyused to implement the channelizing codes.

[0007] Wide band CDMA techniques permit problems such as multipathfading to be more readily overcome and provide a relatively high signalgain. However, some form of signal diversity is also generally providedto further reduce the deleterious effects of fading and additionalproblems associated with acquiring and demodulating signals in thepresence of relative user, or repeater, movement, which along with largedistances causes substantial dynamic changes in path lengths.

[0008] Generally, three types of diversity are used in spread spectrumcommunication systems, including time, frequency, and space diversity.Time diversity is obtainable using repetition and time interleaving ofsignal components, and a form of frequency diversity is inherentlyprovided by spreading the signal energy over a wide bandwidth.

[0009] Space or path diversity is obtained by providing multiple signalpaths through simultaneous links with a user through two or more basestations or antennas, for terrestrial-based repeater systems; or two ormore satellites or satellite beams, for space-based repeater systems.That is, for terrestrial systems signals can be transferred throughmultiple base stations, or more likely, through multiple antennasservicing various cell sectors. For satellite communication systems,path diversity is typically obtained by transferring signals overmultiple paths using either multiple satellites (repeaters) or multipletransponder beams on a single satellite. However as discussed below, thelatter approach is not generally useful.

[0010] Examples of using path diversity in multiple access communicationsystems are illustrated in U.S. Pat. No. 5,101,501 entitled “SoftHandoff In A CDMA Cellular Telephone System,” issued Mar. 31, 1992, andU.S. Pat. No. 5,109,390 entitled “Diversity Receiver In A CDMA CellularTelephone System,” issued Apr. 28, 1992, both assigned to the assigneeof the present invention, and incorporated herein by reference.

[0011] Typical spread spectrum communication systems also contemplatethe use of a “pilot” carrier signal as a coherent phase reference forgateway- or satellite-to-user and base station-to-user links. That is, apilot signal, which typically contains no data modulation, istransmitted by a base station or gateway throughout a given region ofcoverage. A single pilot is typically transmitted by each gateway orbase station for each frequency used, typically referred to as a CDMAchannel, or sub-beam. This pilot is shared by all user terminalsreceiving signals from that source. This provides signals that can bereadily distinguished from each other, also distinguishing between beamsand cells while providing simplified acquisition and tracking.

[0012] Pilot signals are used by subscriber units to obtain initialsystem synchronization, and provide robust time, frequency, and phasetracking of transmitted signals. Phase information obtained fromtracking a pilot signal carrier is used as a carrier phase reference forcoherent demodulation of communication system or user informationsignals.

[0013] Pilot signals are also generally used to gauge relative signal orbeam strength for received communication signals. In many systems, pilotsignals are also generally transmitted at a higher power level thantypical traffic or other data signals to provide a greatersignal-to-noise ratio and interference margin. This higher power levelalso enables an initial acquisition search for a pilot signal to beaccomplished at high speed while providing for very accurate tracking ofthe pilot carrier phase using relatively wide bandwidth, and lower cost,phase tracking circuits.

[0014] As satellites transit in their respective orbits, the beams theyproject onto the Earth move relative to users, periodically changingwhich satellites can provide service for particular users. This occursfor example as satellites come into or disappear from “view”. The sameeffect also occurs between beams in a single satellite, with service forparticular users changing as the beams move across the earth's surface.In addition, mobile users sometimes move relative to beams or satellitepaths, also causing beam coverage or service areas to change. In thesesituations, communication links for signals must be handed off betweenbeams. A similar process occurs for terrestrial cellular systems whereusers move relative to base stations and sectors or sector boundarieswithin cells.

[0015] A basic technique developed to prevent loss of signal andimproved transfer of information is the so-called “soft” handoff schemewhich is described in U.S. Pat. No. 5,101,501, referred to above. Inthis technique, a new link or signal path is established through a newsatellite, or satellite beam, before the existing or old link isdisconnected or discarded. The information (energy) available for agiven communication signal from each path can be combined to provideimproved signal reception, as well as prevent disconnected communicationlinks. This can be done for either the forward link communications fromgateway-to-user terminal, or the reverse link communications from userterminal-to-gateway. For the reverse link, the diversity combiningprocess is accomplished at the gateway or within a centralized controlor switching center.

[0016] Unfortunately, when using soft handoff techniques in satellitecommunication systems several problems arise. While diversity can beused to improve signal characteristics for communication links involvingmultiple satellites, it is not useful for communicating to a userthrough multiple beams on a single satellite. Beams from a singlesatellite have virtually the same path at the same frequency on aforward link, with nearly the same transit time, and have the samefading or interference characteristics. Diversity combining two suchforward link signals provides little benefit, while unnecessarilyconsuming power and adding to the background noise level orinterference.

[0017] Users can also traverse between adjacent beams quickly and moveback and forth along their respective boundaries. If a user is movingalong the Earth's surface perpendicular to the direction of sweep for asatellite spot containing a series of beams, the user might traversebetween two adjacent beams repeatedly. In this situation, a user canswitch between adjacent beams on a frequent basis, especially where thebeams are near the edge of coverage for a satellite spot. In addition,other factors such as low satellite elevation and local terrain orsignal blockage continuously impact signal quality. In this situation,the communication system may be continuously switching between beams ina soft handoff mode to maintain a best communication link.

[0018] A similar process may occur for mobile users moving around insectored cells in terrestrial communication systems. That is, where thecells are subdivided into two or more smaller service areas which arecovered at differing frequencies or using different code spaces. Here,mobile users may travel along or repeatedly cross sector boundarieswithin a cell, depending on such factors as cell and sector size andlocal physical environment. The resulting switching activity may beincreased by the use of techniques meant to otherwise increase cellcapacity. For example, a cell may employ a series of relatively smallsectors or sectors having adjustable sizes to increase capacity oraccommodate certain traffic patterns relative to the cell service area.However, smaller sectors and more sector boundaries increase thelikelihood of more frequent handoffs between sectors. Changing sectorsizes may also shift a user terminal back and forth between adjacentsectors with a minimum amount of physical movement.

[0019] This switching activity tends to consume excessive systemresources in several ways. First, the time spent establishing links andselecting channels, with corresponding signal time, frequency, and phasetracking, error detection, and so forth, consumes signal processingresources which could be applied to other tasks such as signaldemodulation, diversity combining, and decoding. Second, for asubstantial period of time, multiple orthogonal channels in each beamare in use by a single user. That is, orthogonal codes in adjacentbeams, or sectors, are allocated to a single user. Since there are arelatively limited number of such orthogonal channels available in thecommunication system, this decreases effective system capacity. Third,additional power is consumed maintaining each active channel for asingle user, double for two channels, and energy deposited into suchcommunication channels causes interference, which is deleterious tosystem operation.

[0020] Therefore, what is needed is a handoff technique which allows asoft handoff between adjacent beams from a single satellite withdecreased system resources when the user is traversing between suchbeams. The technique should also address soft handoff between adjacentsectors within a cell serviced by a base station or cell-site. Themethod should provide a solution that decreases unnecessary consumptionof system resources while remaining compatible with other soft handoffschemes.

SUMMARY OF THE INVENTION

[0021] In view of the above problems encountered in the art, one purposeof the present invention is to provide a technique for handing off ortransferring communication links between adjacent service areas definedby beams of a single satellite or sectors in a cell, while minimizingutilization of system resources.

[0022] An advantage of the present invention is that soft handoff can beemployed for reverse link signal transfer while being eliminated or usedless frequently and/or for shorter durations on forward link transfers.

[0023] Another purpose of the invention is to reduce switching andcommunication signal tracking and control operations during transfersbetween adjacent service areas for single satellites and cells.

[0024] Another advantage of the invention is that system capacity can beincreased by increasing the general availability of orthogonalchannelizing codes and traffic channels.

[0025] Yet another advantage of the invention is that certain pilotsignal adjustments can be accommodated more accurately, allowingincreased system capacity.

[0026] These and other purposes, advantages, and objects of the presentinvention are realized in a method and apparatus for performing handoffbetween adjacent service areas in a wireless communication system thattransfers communication signals using at least one centralcommunications station which establishes geographical service areas foruser terminals operating within the system. The central station isgenerally either a gateway that establishes adjacent service areas usingsatellite beams from a single satellite, or a single base station thatestablishes adjacent service areas as sectors of a cell.

[0027] A physical transition of a user terminal between two adjacentservice areas, each established by a common central communicationsstation, is detected by determining the signal strength for signalsoriginating from the adjacent service areas. While the user terminalcontinues to use a forward link channel in a first service area, the useof a forward link channel in a second service area is set up. Thisaction is taken when a detected signal strength for the second adjacentservice area at least equals that of the first service area. Once theforward link traffic channel is established in the second service area,its satisfactory operation is confirmed according to a preselectedminimum quality level, based on various known criteria, and the forwardlink for the first service area is disengaged or inactivated. Applicablecriteria are based on known factors, such as on determining if the newchannel has sufficient energy, or a sufficiently low error rate tomaintain a desired level of communication service.

[0028] Preferably either pilot or paging signals associated with theservice areas form the signals used for detecting service areatransitions, and the strength of such signals determines a signalstrength for each service area relative to the user terminal position.The pilot or paging signals are received using at least one userterminal receiver, and their strength is measured using known techniquesand processing elements. The strength of signals from different serviceareas can then be compared, typically by at least temporarily storingone or more measurements for operation on by one or more comparators,control processors, or other known processing elements.

[0029] Preferably, signal strength measurement information istransmitted as part of one of several known types of signals to thecentral station, which receives the measurement information using knownsignal reception means and techniques. The central station then comparesreceived signal strength values and determines relative signalstrengths. The central station may use additional signal informationavailable internally as part of this comparison or in determining signalstrength.

[0030] The central station can then use a communications transmitter totransmit the results of this comparison to the user terminal. At thesame time, the central station can set up a desired new channel throughthe new service area to be used, in accordance with known capacitylimitations, or various channel assignment procedures and schemes. Byperiodically reporting pilot signal measurements to the central station,a need for new channels can be more readily anticipated, allowing somechannels to even be reserved as desired.

[0031] Alternatively, the signal strength measurement information isused by the user terminal to detect and compare the signal levels forthe two adjacent service areas. The user terminal determines that atransition between the service areas is occurring, or that the relativestrength of a signal from a new service area exceeds that currently inuse. The user terminal sends this information to the gateway or basestation, instead of sending signal measurement information. The gatewayagain determines if a new traffic channel can be assigned, and assignsthe new channel, as appropriate to implement the handoff.

[0032] In further aspects of the invention, the presence of adjustedpilot signals is detected. That is, a means is used to detect pilotsignals being received that have had their power adjusted duringtransmission to boost signal strength and compensate for signal roll-offconditions near the edges of beams. When such adjusted pilot signals aredetected, a so-called a compensation factor is derived for each onewhich has substantially the same magnitude as the boost or increaseapplied to the signal. This compensation factor is then applied as anegative adjustment or bias to the signal level during the strengthmeasurements for each such adjusted pilot signal to compensate for theartificial boost in power and arrive at a more accurate non-adjustedstrength determination. This compensation factor or value can be appliedeither at the user terminal or the central station, as desired.

[0033] In addition, the central station can synchronize the timing ofcommunication signals and forward link channels for a user terminalthrough both old and new service areas. This can be done when either thegateway or the user terminal determines that a new forward link channelis desired for the user terminal in a new service area. By usingappropriate signal timing and control elements in the central station,the signal timing can be synchronized so that the forward link of thefirst service area can be disengaged and the use of the forward linkchannel for said second service area commenced at substantially the sametime.

[0034] It is very desirable to prevent undue switching between beams anda corresponding expenditure of system resources. Therefore, in furtheraspects of the invention, a form of hysteresis can be used in which thevalue for at least one pre-selected communication parameter is inspectedon a periodic basis. Any request for a new forward link channel iseither prevented from being generated or blocked from transfer until aminimum change in the monitored value has occurred, since a new forwardlink channel was previously requested. Exemplary parameters are time andsignal energy level. The user terminal can determine when a pre-selectedminimum period of time has passed since a new forward link channel waspreviously requested, or when a pre-selected minimum signal level hasbeen reached by a current service area signal before requesting aforward link channel.

[0035] This can be implemented, for example, by storing signalidentification information for each service area used, up to apredetermined maximum number, in a memory for a predetermined maximumlength of time. Signal identification for any newly detected servicearea is then compared to stored identification information to determineif the same service area is being detected again, and within arestricted period of time. This information can be used by centralstations, gateways or base stations, to limit the amount of inter-beamor inter-sector switching.

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The features, objects, and advantages of the present inventionwill become more apparent from the detailed descriptions set forth belowwhen taken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

[0037]FIG. 1 illustrates an overview of a wireless spread spectrumcommunication system using satellites;

[0038]FIG. 2a illustrates a perspective view of a signal beam patternbetween one of the satellites of FIG. 1 and the surface of the Earth;

[0039]FIG. 2b illustrates a perspective view of a signal beam patternbetween a base station of FIG. 1 and the surface of the Earth;

[0040]FIG. 3a illustrates a theoretical satellite communication signalfootprint with corresponding beam patterns for one of the satellites inFIG. 1;

[0041]FIG. 3b illustrates an exemplary signal footprint and beampatterns for one of the satellites in FIG. 1 with typical beam sizevariations and overlap;

[0042]FIG. 3c illustrates an exemplary signal pattern for a base stationin FIG. 1 with typical theoretical sector boundaries and variations;

[0043]FIG. 4 illustrates user terminal apparatus operating according tothe present invention within the system of FIG. 1;

[0044]FIG. 5 illustrates typical gateway apparatus operating accordingto the present invention within the system of FIG. 1;

[0045]FIG. 6a illustrates an exemplary straight path for a user terminaltraversing adjacent beams;

[0046]FIG. 6b illustrates an exemplary irregular path for a userterminal traversing adjacent beams;

[0047]FIG. 6c illustrates an exemplary irregular path for a userterminal traversing adjacent sectors in a cell;

[0048]FIG. 7 illustrates steps used in the handoff process of thepresent invention for user terminals;

[0049]FIG. 8 illustrates additional steps used in the handoff process ofFIG. 7 when the signal source determines pilot strength; and

[0050]FIG. 9 illustrates additional steps used in the handoff process toaccount for pilot power adjustments and to implement hysteresis.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0051] The present invention is a handoff technique in which systemusers detect transitions between service areas defined by satellitebeams, or sectors in a cell, and request a forward link channel in a newservice area when a detected signal strength for a signal servicing thatarea exceeds one or more predetermined threshold levels. Forward andreverse direction communication links in the current service area aremaintained until the strength of a new service area signal reaches acertain level, and appropriate channel quality is confirmed, as based onvarious known factors. Typically, service area transitions are detectedby receiving new pilot or paging signals associated with new serviceareas, and it is the strength of such signals that is used to determinea relative signal strength of the new beam or sector.

[0052] The present invention is particularly suited for use incommunications systems employing either Earth orbiting satellites, orhighly sectorized cells. However, it will be apparent to those skilledin the relevant art that the concept of the present invention can beapplied to a variety of satellite systems even when not utilized forcommunications purposes. The present invention can also be applied tocells using a variety of cell sectorization schemes, again, even whennot utilized for user communications.

[0053] The preferred embodiment of the invention is discussed in detailbelow. While specific steps, configurations and arrangements arediscussed, it should be understood that this is done for illustrativepurposes only. A person skilled in the relevant art will recognize thatother steps, configurations and arrangements can be used withoutdeparting from the spirit and scope of the present invention. Thepresent invention could find use in a variety of wireless informationand communication systems, including those intended for positiondetermination, and satellite and terrestrial cellular telephone systems.A preferred application is in CDMA wireless spread spectrumcommunication systems for mobile or portable telephone service.

[0054] An exemplary wireless communication system in which the presentinvention is found useful, is illustrated in FIG. 1. It is contemplatedthat this communication system uses CDMA type communication signals, butthis is not required by the present invention. In a portion of acommunication system 100 illustrated in FIG. 1, one base station 112,two satellites 116 and 118, and two associated gateways or hubs 120 and122 are shown for effecting communications with two remote userterminals 124 and 126. Typically, the base stations andsatellites/gateways are components of separate communication systems,referred to as being terrestrial and satellite based, although, this isnot necessary. The total number of base stations, gateways, orsatellites in such systems depends on desired system capacity and otherfactors well understood in the art.

[0055] The terms base station and gateway are also sometimes usedinterchangeably, each being a fixed central communication station, asreferenced above, with gateways being perceived in the art as highlyspecialized base stations that direct communications through satelliterepeaters while base stations (also sometimes referred to as cell-sites)use terrestrial antennas to direct communications within surroundinggeographical regions. Gateways have more ‘housekeeping tasks,’ withassociated equipment, to maintain satellite communication links, and anycentral control centers also typically have more functions to performwhen interacting with gateways and moving satellites. However, thepresent invention finds application in systems using either gateways orbase stations as central fixed communication stations.

[0056] User terminals 124 and 126 each have or comprise a wirelesscommunication device such as, but not limited to, a cellular telephone,a data transceiver, or a paging or position determination receiver, andcan be handheld or vehicle mounted as desired. However, while userterminals are generally viewed as being mobile, it is also understoodthat the teachings of the invention are applicable to “fixed” units insome configurations. User terminals are sometimes also referred to assubscriber units or simply as ‘users’ in some communication systems,depending on preference.

[0057] Generally, beams from base station 112 or satellites 116 and 118cover different geographical areas in predefined patterns. Beams atdifferent frequencies, also referred to as CDMA channels or ‘sub-beams’,can be directed to overlap the same region. It is also readilyunderstood by those skilled in the art that beam coverage or serviceareas for multiple satellites, or antenna patterns for multiple basestations, might be designed to overlap completely or partially in agiven region depending on the communication system design and the typeof service being offered, and whether space diversity is being achieved.

[0058] While only two satellites are shown for clarity, a variety ofmulti-satellite communication systems have been proposed with anexemplary system employing on the order of 48 or more satellites,traveling in eight different orbital planes in Low Earth Orbit (LEO) forservicing a large number of user terminals. However, those skilled inthe art will readily understand how the teachings of the presentinvention are applicable to a variety of satellite system and gatewayconfigurations. This includes other orbital distances andconstellations, for example, those using geostationary satellites wherebeam-switching results mostly from user terminal motion. In addition, avariety of base station configurations can also be used.

[0059] In FIG. 1, some possible signal paths are illustrated forcommunications being established between user terminals 124 and 126 andbase station 112, or through satellites 116 and 118, with gateways 120and 122. The base station-user terminal communication links areillustrated by lines 130 and 132. The satellite-user terminalcommunication links between satellites 116 and 118, and user terminals124 and 126 are illustrated by lines 140, 142, and 144. Thegateway-satellite communication links, between gateways 120 and 122 andsatellites 116 and 118, are illustrated by lines 146, 148, 150, and 152.Gateways 120 and 122, and base station 112, may be used as part of oneor two-way communication systems or simply to transfer messages or dateto user terminals 124 and 126.

[0060] Communication system 100 generally includes one or more systemwide controllers or switching networks 160. Exemplary elements used insuch controllers are mobile telephone switching offices (MTSO), whichinclude interface and processing circuitry for controlling routing oftelephone calls between a public switched telephone network (PSTN) andgateways. Other exemplary equipment includes ground operations controland command centers which provide system-wide control over timing, PNand orthogonal code and frequency assignments, system access, and soforth, for gateways and satellites. A communication link 162 couplingcontrollers 160 to various gateways or base stations can be establishedusing known techniques such as, but not limited to, dedicated telephonelines, optical fiber links, or microwave or dedicated satellitecommunication links.

[0061] As shown in FIG. 2a, satellites in such a communication system(100), including satellites 116 and 118, project beams in “spots” or“footprints” 210 that move over the Earth's surface in accordance withsatellite orbital motion. The satellite footprint consists of one spot210 formed by a series of separate beams 212, or sub-beams, projected ina generally circular pattern. Here, spot 210 is formed with one centralbeam in the middle surrounded by a series of beams 212. However, avariety of patterns, beams and beam sizes can be used, as would be knownto one skilled in the art. As discussed further below, a user may movefrom a position X in one beam 212 to a position Y in a neighboring beam212 along a path illustrated by line 214. This can occur as a result ofeither user terminal or beam movement or a combination of both.

[0062] Typically, communication system 100 subscribers or users mayutilize signal paths through satellites 116 and 118 when they areelevated anywhere from 10 degrees or more above a horizon measuredrelative to the user terminal seeking communication service. The angleat which useful communication occurs is, however, dependent upon whetheror not there are obstructive or attenuating objects in the path, andknown system requirements or desires for minimum error rates andinterference.

[0063] As shown in FIG. 2b, base stations or cell cites in such acommunication system (100), including base station 112, project beams orsignals within a cell 220 covering a predetermined service area on theEarth's surface in accordance with signal strength and local terrain.Cell 220 consists of one overall coverage area formed by a series ofseparate beams or signals that create sectors 222, projected in agenerally wedge shaped patterns. Here, cell 220 is formed using a seriesof six sectors 222, not all having the same area or size. However, avariety of patterns, sectors, and sector sizes can be used, as would beknown to one skilled in the art. As discussed further below, a user maymove from a position X in one sector 222 to a position Y in aneighboring sector 222 along a path illustrated by line 224. This occursas a result of either user terminal movement or changing sector coverageor a combination of both.

[0064] Exemplary beam and sector patterns are illustrated in furtherdetail in FIGS. 3a, 3 b, and 3 c. FIGS. 3a and 3 b illustrate satellitecommunication system signals projected onto the surface of the Earth,while FIG. 3c illustrates a coverage pattern for a generally circularsectorized cell. However, other patterns of elongated or irregular shapecan be employed within the teachings of the invention, as desired.

[0065] In FIG. 3a, a series of beams B1-B16 are shown in a generallycircular pattern or spot 210. Spot 210 is formed with one central beamB1 in the middle surrounded by six beams B2-B7, and then nine additionalbeams B8-B16. This is an ‘ideal’ pattern which is illustrated as havingprecisely straight edges and non-overlapping regions of coverage betweenadjacent beams. The adjacent beams in this example operate in the samefrequency band and multiple sub-beams form the same pattern withcorresponding regions of coverage overlaid on this pattern, eachoperating at different frequencies. Those skilled in the art arefamiliar with this type of pattern and the frequency and PN codeassignments used to form such patterns.

[0066] As would be readily apparent to those skilled in the art, actualbeams are more circular or elliptical in shape, and form more elongatedor irregularly shaped patterns as they are projected by satellitetransponders or antenna systems. The beams or sub-beams also createoverlapping regions of coverage, with beam energies generally beingtailored at transmission, to decrease somewhat rapidly near the edges orboundaries, to decrease overlapping signal coverage. A resulting type ofpattern more closely representative of these effects is illustrated inFIG. 3b, where each of the beams is shown as a more circular spot withadjacent beams having regions of slight overlap.

[0067] In FIG. 3c, a series of sectors S1-S6 are shown in a generallycircular pattern or cell 220. This cell is illustrated as havingirregular edges as a result of how the signals are projected bytransponders or antenna systems and the impact of local terrain orstructures, as known in the art. As illustrated, the sectors need not beuniform in size, and may even have their respective coverage areasadjusted during operation of the communication system. The sector beamsor signals also create overlapping sector boundaries or regions ofcoverage between adjacent sectors, with beam energies generally beingtailored at transmission, to decrease more rapidly near the edges orboundaries, to decrease overlapping signal coverage. The overlappingboundaries are shown using solid and dashed lines for adjacent sectorboundaries. The adjacent sectors in this example each use different PNcodes or code offsets in a manner similar to the satellite sub-beams.Those skilled in the art are familiar with these types of patterns andthe frequency and PN code assignments used to form such patterns.

[0068] An exemplary transceiver 400 for use in a user terminal 126 toacquire signals or channels in beams B1-B16 is illustrated in FIG. 4.Such transceivers are known in the art and discussed in the patentsreferenced above, such as U.S. Pat. No. 5,109,390.

[0069] Transceiver 400 uses at least one antenna 410 for receivingcommunication signals which are transferred to an analog receiver 414,where they are downconverted, amplified, and digitized. A duplexerelement 412 is typically used to allow the same antenna to serve bothtransmit and receive functions. However, some systems employ separateantennas for operating at different transmit and receive frequencybands.

[0070] The digital communication signals output by analog receiver 414are transferred to at least one digital data receiver 416A andpreferably at least one digital searcher receiver 418. Additionaldigital data receivers 416B-416N can be used to obtain desired levels ofsignal diversity or receive multiple signals, depending on theacceptable level of unit complexity, as would be apparent to one skilledin the relevant art. Additional searcher receivers can also be used forimplementing more complex signal acquisition or searching techniques.

[0071] At least one user terminal control processor 420 is coupled todata receivers 416A-416N and searcher receiver 418. Control processor420 provides, among other functions, basic signal processing, timing,power and handoff control or coordination, and selection of frequencyused for signal carriers. Another basic control function often performedby control processor 420 is the selection or manipulation of PN codesequences or orthogonal functions to be used for processingcommunication signal waveforms. Control processor 420 signal processingcan include a determination of relative signal strength and computationof various related signal parameters. Such computations of signalparameters, such as timing and frequency may include the use ofadditional or separate dedicated circuitry to provide increasedefficiency or speed in measurements or improved allocation of controlprocessing resources. For example, in FIG. 4 a signal strength measuringelement 421 is shown for using certain information available in theanalog receiver to determine the signal strength or power for theoverall received analog signal. Measuring element 421 is also shownusing outputs of, or data available from, the digital data and searcherreceivers for measuring the energy or power in specific signals beingreceived or demodulated.

[0072] Outputs for data receivers 416A-416N are coupled to remainingdigital baseband circuitry 422 within the user terminal. User digitalbaseband circuitry 422 comprises processing and presentation elementsused to transfer information to and from a user terminal user. That is,signal or data storage elements, such as transient or long term digitalmemory; input and output devices such as display screens, speakers,keypad terminals, and handsets; A/D elements, vocoders and other voiceand analog signal processing elements; etc., all form parts of thesubscriber baseband circuitry using elements well known in the art. Ifdiversity signal processing is employed, user digital baseband circuitry422 can comprise a diversity combiner and decoder. Some of theseelements may also operate under the control of, or in communicationwith, control processor 420.

[0073] When voice or other data is prepared as an output message orcommunications signal originating with the user terminal, user digitalbaseband circuitry 422 is used to receive, store, process, and otherwiseprepare the desired data for transmission. User digital basebandcircuitry 422 provides this data to a transmit modulator 426 operatingunder the control of control processor 420. The output of transmitmodulator 426 is transferred to a power controller 428 which providesoutput power control to a transmit power amplifier 430 for finaltransmission of the output signal from antenna 410 to a gateway or basestation.

[0074] Information or data corresponding to one or more measured signalparameters for received communication signals, or one or more sharedresource signals, can be sent to the gateway using a variety oftechniques known in the art. For example, the information can betransferred as a separate information signal or be appended to othermessages prepared by user digital baseband circuitry 422. Alternatively,the information can be inserted as predetermined control bits bytransmit modulator 426 or transmit power controller 428 under control ofcontrol processor 420, using known “puncturing” or multiplexingtechniques.

[0075] Data receivers 416A-N and searcher receiver 418 are configuredwith signal correlation elements to demodulate and track specificsignals. Searcher receiver 418 is used to search for pilot signals, orother relatively fixed pattern strong signals, while data receivers416A-N are used to demodulate other signals associated with detectedpilot signals. For purposes of determining signal strength, however, adata receiver 416 can be assigned to track the pilot signal afteracquisition to accurately determine the ratio of signal chip energies tosignal noise. The pilot signal chip energies are integrated overpredetermined intervals, such as symbol periods, to formulate pilotsignal strength. Therefore, the outputs of these units can be monitoredto determine the energy in or frequency of the pilot signal or othersignals. These receivers also employ frequency tracking elements thatcan be monitored to provide current frequency and timing information, tocontrol processor 420 for signals being demodulated.

[0076] An exemplary transmission and reception apparatus 500 for use ina gateways 120 and 122 is illustrated in FIG. 5. Such apparatus is knownin the art and discussed in the patents referenced above. For example,additional details on the operation of this type of apparatus are foundin U.S. Pat. No. 5,103,459, issued Apr. 7, 1992, entitled “System AndMethod For Generating Signal Waveforms In A CDMA Cellular Telephone,”assigned to the same assignee as the present invention and incorporatedherein by reference.

[0077] The portion of gateway 120, 122 illustrated in FIG. 5 has one ormore analog receivers 514 connected to an antenna 510 for receivingcommunication signals which are then downconverted, amplified, anddigitized using various schemes well known in the art. Multiple antennas510 are used in some communication systems. Digitized signals output byanalog receiver 514 are provided as inputs to at least one digitalreceiver module, indicated by dashed lines generally at 524.

[0078] Each digital receiver module 524 corresponds to signal processingelements used to manage communications between one user terminal 124,126 and a base station 112 or a gateway 120, 122, although certainvariations are known in the art. One analog receiver 514 can provideinputs for many digital receiver modules 524, and a number of suchmodules are typically used in gateways 120, 122 to accommodate all ofthe satellite beams and possible diversity mode signals being handled atany given time. Each digital receiver module 524 has one or more digitaldata receivers 516 and preferably at least one digital searcher receiver518. Searcher receiver 518 generally searches for appropriate diversitymodes of signals other than pilot signals. Where implemented in thecommunication system, multiple data receivers 516A-516N are used fordiversity signal reception.

[0079] The outputs of digital data receivers 516 are provided tosubsequent baseband processing elements 522 comprising apparatus wellknown in the art and not illustrated in further detail here. Exemplarybaseband apparatus includes diversity combiners and decoders to combinemultipath signals into one output for each subscriber. Exemplarybaseband apparatus also includes interface circuits for providing outputdata, typically to a digital switch or network. A variety of other knownelements such as, but not limited to, vocoders, data modems, and digitaldata switching and storage components may form a part of basebandprocessing elements 522. These elements operate to control or direct thetransfer of data signals to one or more transmit modules 534.

[0080] Signals to be transmitted to user terminals are each coupled toone or more appropriate transmit modules 534. A typical gateway uses anumber of such transmit modules 534 to provide service to many userterminals 124, 126 at a time, and for several satellites and beams at atime. A base station may also use a number of such modules, althoughbase stations tend to group transmit and receive functions more closelytogether in modem structures. The number of transmission modules 534used by gateway 120, 122 is determined by factors well known in the art,including system complexity, number of satellites in view, subscribercapacity, degree of diversity chosen, and the like.

[0081] Each transmit module 534 includes a transmit modulator 526 whichspread-spectrum modulates data for transmission. Transmit modulator 526has an output coupled to a digital transmit power controller 528, whichcontrols the transmission power used for the outgoing digital signal.Digital transmit power controller 528 applies a minimum level of powerfor purposes of interference reduction and resource allocation, butapplies appropriate levels of power when needed to compensate forattenuation in the transmission path and other path transfercharacteristics. A PN generator 532 is used by transmit modulator 526 inspreading the signals. This code generation can also form a functionalpart of one or more control processors or storage elements used ingateway 122, 124, or base station 112.

[0082] The output of transmit power controller 528 is transferred to asummer 536 where it is summed with the outputs from other modulators ortransmit power control circuits. Those outputs are signals fortransmission to other user terminals 124, 126 at the same frequency andwithin the same beam as the output of transmit power controller 528. Theoutput of summer 536 is provided to an analog transmitter 538 fordigital-to-analog conversion, up-conversion to the appropriate RFcarrier frequency, further amplification and output to one or moreantennas 540 for radiating to user terminals 124, 126. Antennas 510 and540 may be the same antennas depending on the complexity andconfiguration of the system.

[0083] At least one gateway control processor 520 is coupled to receivermodules 524, transmit modules 534, and baseband circuitry 522; theseunits may be physically separated from each other. Control processor 520provides command and control signals to effect functions such as, butnot limited to, signal processing, timing signal generation, powercontrol, handoff control, diversity combining, and system interfacing.In addition, control processor 520 assigns PN spreading codes,orthogonal code sequences, and specific transmitters and receivers foruse in subscriber communications.

[0084] Control processor 520 also controls the generation and power ofpilot, synchronization, and paging channel signals and their coupling totransmit power controller 528. The pilot channel is simply a signal thatis not modulated by data, and may use a repetitive unchanging pattern ornon-varying frame structure type input (pattern) into transmit modulator526. That is, the orthogonal function, Walsh code, used to form thechannel for the pilot signal generally has a constant value, such as all1's or 0's, or a well known repetitive pattern, such as a structuredpattern of interspersed 1's and 0's. This effectively results intransmitting only the PN spreading codes applied from PN generator 532.In addition, a pilot signal is non-power controlled. That is, the pilotsignal is transmitted at a preselected fixed power level, which is notvaried so that accurate measurements of signal power are achieved byuser terminals.

[0085] While control processor 520 can be coupled directly to theelements of a module, such as transmit module 524 or receive module 534,each module generally comprises a module-specific processor, such astransmit processor 530 or receive processor 521, which controls theelements of that module. Thus, in a preferred embodiment, controlprocessor 520 is coupled to transmit processor 530 and receive processor521, as shown in FIG. 5. In this manner a single control processor 520can control the operations of a large number of modules and resourcesmore efficiently. Transmit processor 530 controls generation of, andsignal power for, pilot, synchronization, paging signals, and trafficchannel signals, and their respective coupling to power controller 528.Receiver processor 521 controls searching, PN spreading codes fordemodulation and monitoring received power.

[0086] For certain operations, such as shared resource power control,gateways 120 and 122 receive information such as received signalstrength, frequency measurements, or other received signal parametersfrom user terminals in communication signals. This information can bederived from the demodulated outputs of data receivers 516 by receiveprocessors 521 or receive power measuring elements 523. Alternatively,this information can be detected as occurring at predefined locations inthe signals being monitored by control processor 520, or receiveprocessors 521, and transferred to control processor 520. Controlprocessor 520 uses this information (as described below) to control thetiming and frequency of signals being processed as well as theassignment of digital receivers for user signals.

[0087] Returning now to FIG. 2a, if a user terminal or subscriber unitresiding initially in a region serviced or covered by beam B10,traverses to a region serviced by beam B15, because of either satelliteor terminal motion, any active or established communication link needsto be handed off between the two beams to avoid disruption ofcommunications. Actually, in this situation, there are several handoffsthat take place between any two adjacent beams at a time, as severalbeams (B10, B2, B1/B7, B6) are traversed in succession. This is shown inmore detail in FIGS. 6a and 6 b, where only a few beams are shownadjacent to or along the perceived path for user terminal 122.

[0088] In FIG. 6a, user terminal 122 travels along a straight path 610from point X to point Y. In FIG. 6b, a variable path 620 followed byuser terminal 122 is more irregular, moving from point X to point Y,traversing an additional beam B16. The path will depend on a variety ofknown factors such as speed and direction of movement of the userterminal along the surface of the Earth relative to the satellite, ifmoving, as well as the orbit of the satellite. This is a perceived pathor projected change of location for the user terminal relative to thebeam pattern. If the user terminal is at rest on the Earth, a generallystraight path results as the beams sweep by the user terminal, except asaltered by localized satellite movements. For example, it is well knownthat satellite orientation may be changed from time to time, such as byadjusting yaw, to account for seasonal changes in Earth and Sunpositions or alignments. User terminal motion increases or decreases therate of change along the path for movement parallel to the satelliteorbit direction, and creates irregularities when directed at angles tothe orbital plane. Regardless of the shape of the path, the generalprinciples of the invention and its application are the same.

[0089] As shown in FIG. 6a, user terminal 122 crosses between two beams,initially B10 and B2. In the vicinity of the beam crossover, atransition region is entered in which two adjacent beams are present inthe location of the user terminal. That is, in this region, a userterminal can detect the presence of the pilot signals for both beams. Ina traditional cellular handoff scheme, the user terminal uses a searcherreceiver to acquire the new pilot signal as it is encountered and adigital receiver is assigned to demodulate signals associated with thatpilot, so that a ‘soft handoff’ type communication link can beestablished. After, the new link is established, the user terminal waitsuntil it moves out of the beam coverage for the previous pilot signal(B10) and then drops the link related to that pilot signal.

[0090] Unfortunately, as discussed above, unlike typical multipathreception, there is no benefit in receiving signals using two of thesebeams on the forward link. In typical diversity signal reception, thesignals to be combined are received over markedly different signalpaths, either from different satellites or reflections from surfaces,and so forth. In that situation, the propagation paths are differentenough in terms of time, attenuation, and other path effects, to allowgain from combining. However, for single satellite transfer of multiplebeams of signals, the signals are transferred over virtually identicalsignal paths and the transit time is very nearly the same. Therefore,from a timing and phase point of view, little is to be gained, fordiversity combining these signals.

[0091] In fact, diversity reception of two beams for forward linkcommunication between a satellite and a user terminal can degrade systemperformance in several ways. This process involves excessive use ofavailable resources. First, power is required in each beam for thesatellite to transfer signals to the user terminal. Second, for systemsutilizing orthogonal codes, at least one code is used in each beam forthe user terminal. However, where there is no gain in signal processing,this represents lost power for the satellite and loss of use of a code.This translates to a decrease in system capacity, and unnecessarypotential signal interference.

[0092] A corresponding illustration is provided in FIG. 6c for cell 220.Here, a variable or irregular path 630 is followed by user terminal 122in moving from point X to point Y, traversing sectors S3, S4, S5, S6,and S1. The path depends on a variety of known factors such as speed anddirection of movement of the user terminal, as well as any changes insector boundaries. In FIG. 6c, the overlapping boundaries are againshown using solid and dashed lines. Regardless of the shape of the path,the general principles of the invention and its application are thesame. As in the case of multiple satellite beams, there is generallylittle if any benefit in receiving signals using two of these sectors onthe forward link, except in certain circumstances.

[0093] The present invention takes advantage of some of the propertiesof sectors, satellite beams, and communication links and their control,to improve the handoff scheme for beam-to-beam or sector-to-sectortransitions. The present invention decreases the power and coderesources required for each user terminal undergoing such transitions,while maintaining ‘soft’ communication links. A flow chart representingof the steps used to implement handoff processing according to oneembodiment of the present invention is illustrated in FIG. 7.

[0094] It is readily understood that due to the shape of the beams,there is a power or energy distribution across the beams that placeslower power near a beam edge. Generally, this means that one or theother of the two beam signals is largest. Therefore, the transition fromone beam to the next results in a gradual or rapid (depending ontransition speed) build up in received power for one beam and acorresponding decrease in power from the other. That is, an increase ordecrease in signal strength for received pilot signals is detectedduring a transition between the two adjacent beams. Where two or morebeams intersect or overlap completely, the power of the beams may alsobe balanced substantially the same. The same effect is observed foradjacent sectors in a cell.

[0095] As seen in a step 710 of FIG. 7, a user terminal detects andacquires a pilot signal at some point in time, and uses this signal toestablish a forward communication link. This could occur when the userterminal first commences communication, such as when starting at point Xin beam B10, in FIG. 6a, or sector S4 in FIG. 6c. If several pilotsignals are detected by a user terminal, generally the strongest signalis chosen for further processing. However, those skilled in the art willreadily understand that other basis for selecting a pilot signal can beused as desired within the communication system, when first establishinga communication link. For, example some pilot signals could represent ororiginate from gateways which a particular user terminal is not allowedto communicate with for various technical or procedural reasons.

[0096] As discussed above, the use of pilot signals represents onepreferred mode of operation for the invention, and other strong sharedresource signals such as paging signals may also be used as desired.

[0097] The first acquired pilot signal is used, in a step 712, as atiming and phase reference to acquire and demodulate forward linkcommunication signals associated with that pilot, or the base station orgateway transmitting that pilot. However, as the user terminal or thesatellite moves, or as cell or beam boundaries are adjusted, at leastone new pilot signal is detected in a step 714, as the user terminalapproaches a beam or sector boundary or edge. A user terminal searcherreceiver generally acquires this new pilot signal (step 714) and itsrelative signal strength is determined in a step 716, as compared tothat of the previously selected pilot signal (step 710). The strength ofthe new pilot will either grow larger and larger as the user terminalcrosses further into the new beam, or it will decrease as the pathchanges to enter another beam or move farther interior of the originalbeam.

[0098] As long as the signal strength of a newly detected pilot signalis less than that of the previous or already in use pilot signal at thispoint, the new pilot is not used to establish a new communication linkor to set up a channel in the new beam. However, the new pilot signalstrength can be compared to a predetermined threshold power level in anoptional step 720. When the new pilot signal reaches this power level,which is still less than that of the previously chosen or in use pilot,the user terminal informs the communication system, or a particulargateway or base station in a step 722. The user terminal can simplyreport the signal strength measurement or that it exceeds the threshold,and allow the gateway to decide when a beam or sector transition isoccurring. Alternatively, the user terminal makes a determination andreports that a transition appears to be approaching and requests a newchannel, depending on user terminal or system complexity.

[0099] There is no requirement for reserving a forward link channel onthe beam although this may be preferred where system capacity issubstantially occupied and a channel will be needed to prevent calltermination. This type of action can be used for ‘priority’ users wheredesired to maintain links. Channel reservation generally means that anorthogonal code is reserved for use by that user terminal, or at leastassigned a priority for its use.

[0100] This first threshold power level is generally established at afew dB less than the strength of the current in use pilot, to minimizesignal processing for brief excursions into the edge of a new beamcoverage region. Those skilled in the art will readily understand how toselect a threshold value based on the desired amount of beam transitionto ignore, and availability of resources in the communication system.This threshold can be a static value or dynamically changeable. Thevalue can be updated as part of the initial system communication withthe user terminal, or on a periodic basis, and stored in a memoryelement for future use by a user terminal controller.

[0101] At some later time, the user terminal determines that thestrength of the new pilot signal is at least equal to that of theprevious pilot signal, in a step 730. At this point, the user terminaltransmits this information or a channel request to the gateway or basestation so that a new forward link communications channel is set up forthe user terminal. Alternatively, an already reserved channel (from step722) is now used. This request may be processed by or through thecentral switching station or a ground operations control type facilityas previously discussed above. In addition, the setup of forward linkchannels need not occur at an exact equality in signal strength, and alower “threshold” type value for the new pilot signal strength can bechosen as desired. Again, depending on allocation of system resources.Those skilled in the art of designing communication systems are familiarwith the criteria used for selecting this threshold level.

[0102] At this point, the new channel in the new beam is selected foruse in a step 732, and the user terminal communicates both over achannel associated with the older pilot and beam, and over a channelassociated with the new pilot and beam. This is similar to moreconventional soft handoff signal processing on the forward link. Thecommunication system, through the gateway or base station, is informedof the use of these two communication links or paths by the userterminal.

[0103] However, as soon as the gateway receives confirmation, in a step734, from the user terminal that the forward traffic signal is beingreceived satisfactorily from the new beam (channel), the previous beam(channel) signal is taken down, inactivated or dropped, in a step 736.That is, the first beam is no longer used for communication on theforward link with the user terminal. However, in some embodiments theprevious forward link channel may still be reserved for use for someperiod of time, in case the user terminal needs to switch back. Thisprocess results in what can be termed a “quick”, “fast”, or “high-speed”soft handoff.

[0104] The return link signal in any beam is held as long as it provesuseful in processing signals. When the return link reception in any beamor sector is too weak, attenuated, or significantly blocked to provide auseful signal path, it is dropped by the gateway or base station. Thereturn and forward links may be established through separate beam orsector configurations that differ significantly in coverage area orshape. Therefore, so the use of new and termination of old channels orbeams for these two link directions occur independently of each otherand may differ substantially.

[0105] Generally, confirmation step 734 involves determining certainwell known attributes or criteria of the communication signals beingreceived. For example, determining if the signals have sufficientenergy, low enough error rates, and so forth, to support a desired levelof communications. This determination can occur in a very short timespan. As an example, confirmation can be accomplished using known signalparameter examination techniques in the user terminal, or by usingpreselected test data or patterns in signals transmitted to the userterminal which are retransmitted to the central station for receipt andanalysis.

[0106] In typical satellite communication systems and under normalconditions, confirmation occurs after a few frames of data have beentransferred to the user terminal. With a typical data frame in suchsystems being on the order of 20 ms in length, the total time two beamsare in use is on the order of 20-80 ms for measuring signal quality,plus some additional time to account for signal delay through thesatellite (around 10 ms or more). Little or no delay is generallyinvolved for sectored cellular systems.

[0107] There are several approaches to determining and utilizing thepilot signal strength measurements. The user terminal can try todetermine the strength of each pilot and compare them by eithermeasuring each separately in a “direct” or absolute power sense, or“indirectly” by trying to measure a relative difference upon receipt.

[0108] For example, as previously shown in FIG. 4, the amount of energyin a pilot signal can be determined from information or measurementsavailable in searcher 418 and receivers 416A-N, using measuring element421 and control processor 420. The same measurement can be taken for twobeams, sectors, or pilots and stored in data storage element 432 betweencomparison operations, as desired. Searcher receiver 418, is generallytime shared, or switched between pilots signals, or additional receiversare used (416 or 418) for the strength measurements.

[0109] Unfortunately, various path, frequency, and transmission factorswhich may be known by the gateway, or base station, can effect theincident pilot power from beam-to-beam in a manner that makes measuringindividual pilot signals inaccurate. In addition, computing or otherwisedetermining relative signal strengths, and monitoring changes or trendsin pilot signal power can consume more resources than are sometimesdesirable to provide in a user terminal.

[0110] One solution to these problems is to have the gateway or basestation determine the relative and absolute pilot signal strengths frominformation provided by the user terminals. This is a preferred approachbecause the decision can be handled very efficiently by the gateway, orbase station, communicating with the user terminal. In this approach,the user terminal simply reports the level of signal strength beingreceived or a relative value, and changes being experienced. The userterminal can also report when signals are above a certain predeterminedthreshold. This process is shown in FIG. 8, where the first thresholdtest has been omitted.

[0111] In FIG. 8, as before, a user terminal measures the pilot signalstrength in a step 810. This is generally accomplished by integratingreceived pilot signal chip energies over a preselected time interval,such as a symbol period, in a data receiver. This information isgenerally already available as part of various known signal demodulationand tracking schemes used by user terminals. The information is thentemporarily stored, as desired, and either embedded in or appended toother communication signals or transferred as a separate pilotinformation signal to the signal source, either a gateway or basestation, in a step 812.

[0112] Gateways and base stations receive such signals containing signalstrength information in a step 814, and extract the data, usingtechniques known in the art. The information is either automatically oreasily associated with corresponding user terminals and beams. Thegateway then uses this information, along with known transmission powerlevels and relative differences for pilot signals being transmitted in astep 830 to determine the relationships between pilot signals beingdetected or received by the user terminal. That is, to see if the newpilot signal strength exceeds the old. This allows the gateway or basestation to determine relative power levels and when beam or sectorboundaries are being traversed. This information can then be transmittedback to the user terminal as part of various known signals in a step831.

[0113] The gateway establishes a new channel for the user terminal touse in a step 832, having determined when one is desired, in accordancewith known capacity limitations, or various channel assignmentprocedures and schemes. The user terminal will then confirm properoperation of the new channel as before in a step 834, or the gateway canuse certain known feedback mechanisms or predefined transmit-and-receivetest signals to confirm channel operation, before dropping the oldchannel in a step 836. Depending upon the level of signalsynchronization established in setting up the new channel, step 834 canbe optional, as discussed below. This then is a “passive” handofftechnique.

[0114] The gateway or base station can receive periodic reports of pilotsignal strength from user terminals, either in response to transmittedmessages requesting such information, or at preselected reportingintervals. The gateway can update and maintain signal strengthinformation to predict when user terminals approach various coveragearea boundaries.

[0115] An advantage of this approach is that any computation resourcesare limited in terms of apparatus and processing time consumed by a userterminal. Resources can be more easily and cost effectively implementedin base stations and gateways. Another advantage of this approach isthat it allows an alternative embodiment that can be referred to as a“firm” or “synchronized” soft handoff technique.

[0116] Because the gateway or base station is maintaining data on pilotsignal strength from each user terminal, transitions across beam andsector boundaries can be detected very accurately and quickly.Therefore, the gateway can be fully prepared to communicate with a userterminal on multiple beams or sectors (adjacent) to allow rapid changingof channels or channel assignments for a user terminal. The multipleforward link communication paths are fully controlled by the gateway, incombination with central control centers, and all synchronization,timing, and code use issues can be completely resolved in advance ofwhen a handoff to a new beam or sector is desired. Therefore, thegateway can switch the user terminal communications link or path anddrop the use of the traffic signal associated with the first pilot andassociated signals or beam virtually instantly.

[0117] Another problem may occur as a result of “roll-off” near beam orcell edges or boundaries. As with any signal, but more importantly herethe pilot signal, there is an increasingly sharp drop off near the outeredges of a beam. This is a natural result of the power versus distancerelationship for signals, as well as beam forming systems. In satellitesystems, the impact is exaggerated in outer beams in the satellite spot.That is, due to the larger displacement of these beams from a centralarea, the rate of decrease in signal amplitude at outer edges of thebeams may be much more noticeable than for inner beams.

[0118] Roll-off is also increased or exaggerated by certain well knownPower Field Density (PFD) requirements or restrictions placed onsatellite signals. In order to reduce certain types of signalinterference, a limitation is placed on the power density of signalsprojected from satellites. This limitation has its greatest impact nearthe far edges of satellite spots, and some form of compensation isgenerally implemented in the beams near the outer edges. Thiscompensation further reduces incident power in these areas in order tostay within dictated guidelines. Unfortunately, such adjustments greatlyincrease the rate of power roll-off.

[0119] Unfortunately, reduced power in pilot signals also reduces thatability to use them to demodulate paging, traffic, and other signals.Since there is a desire to maintain or increase system capacity acrossthe total service area of the beams, one technique to counter signaldrop-off is to boost the pilot signal power as it is directed to outerbeams or beam boundaries. This can be referred to as a “pilot adjust”technique, and provides a corresponding improvement in signal reception,tracking, demodulation and so forth increasing the number of systemusers near boundaries.

[0120] However, the use of pilot boosting or level adjustments of anytype, including decreasing levels where desired, masks the true beam andsector boundaries. That is, what is normally thought of as “true” beamboundaries will be virtually changed or shifted for a system that usespilot signal strength to account for or detect such boundaries. If thepilot signal level is boosted to have an artificially compensated orstronger level near a particular boundary, the detection mechanismincorrectly determines that a boundary is either closer or farther awaythan it should, based on typical signal roll-off, depending on directionof travel and pilot level controls in adjacent beams.

[0121] Where “pilot adjustment” is utilized, the handoff method of thepresent invention can employ a “pilot-adjust” command or designationtechnique to allow the user terminal to more accurately and correctlydetermine the relative position of beam and sector boundaries. That is,the use of “pilot adjustment” is denoted as part a communication signalsent to the user terminal for each corresponding beam. This can be doneby again embedding or appending a command as part of a paging or trafficsignal, or at certain locations within pilot signals and so forth. Thisinformation allows a user terminal to compensate to some degree for thepilot boost being used. This approach can be made more useful by alsoproviding some relative indication of the amount of boost used, whichcan occur in predefined steps if desired.

[0122] In the alternative, where the gateway or base station iscomputing various pilot strength relationships, the pilot boostinformation is already available within the gateway or base station foreach beam. This process can also allow some adjustment to the pilotadjustment itself, if a large number of user terminals are makingtransitions and there are any problems.

[0123] Use of such “pilot-adjust” commands and processing is shown inFIG. 9, where a pilot signal level adjustment process 910 (dashedoutline) is shown occurring between steps 716 and 730, normally used tomeasure and compare pilot signal levels. Process 910 can occur before orafter first threshold test step 720 in FIG. 7, or strength determinationstep 816 shown in FIG. 8, as well.

[0124] In process 910, a determination is made in a step 912 as towhether or not the pilot has been adjusted. The step 912 determinationuses information available to either a user terminal, or gateway andbase station. Where there is no pilot adjust, signal processing afterstep 912 occurs as before, without benefit of any correcting adjustment.Where no information is available regarding pilot adjustment, the answerobtained in step 912 also indicates no adjustment. Where the pilot isknown to have been adjusted, the level of the pilot is readjusted orcounter-adjusted in a step 914 to make a more accurate comparisonpossible in steps 730 and 830. The amount of adjustment imparted to thepilot may be specified in advance or use dynamically varyinginformation.

[0125] The above embodiments of the new handoff process result in onlyone of the beams being used for forward link signals, with correspondingchannel codes and satellite energy, a majority of the time. Two beamsare used for only a very brief time, while maintaining the advantage ofa soft handoff approach to prevent loss of communications. The operationof the present invention leads to what can be termed as either a “quick”soft handoff technique or a “firm” handoff technique.

[0126] While the above embodiments represent improvements over currenthandoff signal processing techniques, there is one occasion when somesystem resources may still be wasted. This situation occurs when a userterminal path is directed along a common chord for two neighboringbeams. That is, where the user terminal is traversing a path that placesthe user terminal substantially equidistant from the boundaries of two(or more) neighboring beams being traversed. This also occurs where auser terminal is substantially equidistant from the boundaries of twosectors. A similar problem also occurs when a course followed by theuser terminal repetitively crosses adjacent boundaries on a relativelyshort time scale.

[0127] These processes are illustrated in FIGS. 6a, 6 b, and 6 c. InFIG. 6a, a portion 612 of the user terminal path 610 is equally balancedbetween the adjacent beam boundaries. In FIGS. 6b and 6 c, portion 622of path 620, and portions 632 of path 630 are shown wandering back andforth between adjacent beam and sector boundaries.

[0128] In these situations, the user terminal may never have a clearlystronger pilot signal, or the second pilot strength does notconsistently exceed the first for more than brief periods. In both ofthese instances, the user terminal may switch back and forth between twopilots and beams or sectors, as they just meet the threshold criteria,but only for brief periods of time, on the order a few seconds. Thiscauses the user terminal and gateways or central controllers to consumeprecious processing time in switching channels and beams, andredirecting communication signals. In addition, frequent shiftingbetween beams or sectors, reduces the period of single beam coverage,effectively producing the current problematic soft handoff scheme.

[0129] To minimize the inefficient use of system resources, and preventa user terminal near a transition point or within a transition regionfor extended periods of time from changing beams frequently, a form ofhysteresis can be built into the pilot signal processing. The use ofhysteresis in illustrated in FIG. 9, where a hysteresis process 920 hasbeen inserted between pilot level determination step 730 and beam usestep 732. As will be readily apparent to those skilled in the art,process 920 can also occur after comparison determination step 730 inFIG. 7, or determination step 830 shown in FIG. 8.

[0130] Hysteresis process 920 can be accomplished for example byrequiring in a step 912 that the user terminal use a current pilotsignal, or associated beam or sector channels, for a minimum length oftime, subject to certain preselected minimum signal strength orpotential link loss exceptions. As long as the minimum time has passed,the terminal is allowed to switch beams and pilots and proceed asbefore. However, if the time test fails, the new beam cannot be selectedat this point, in spite of passing other threshold tests.

[0131] In some embodiments, each pilot signal being used can beidentified and its identification or relevant characteristics recordedby the user terminal controller in some memory location. For example, aspecific spreading code being used, or some other known beam or pilotspecific identification feature, such as predefined beam “IDs”, can bedetected. This information can be used by a user terminal, such asthrough a searcher receiver, to determine if the same pilot is beingdetected again within a short period of time. The user terminal can useknown internal clocks or timing circuits along with memory elements torecord the length of time a particular pilot or corresponding trafficsignal has been in use.

[0132] Therefore, steps can be taken to minimize the frequency withwhich or minimum time interval over which a beam or sector can bere-selected. The period of time for which a beam or sector must be usedbefore another can be selected, or minimum time before a pilot selectioncan be “repeated” can be determined in advance by communication systemdesigners from the overall amount of system resources expected to beavailable, and other known criteria. The user terminal is provided thisinformation when setup to operate within the system, but thisinformation may be updated as part of various system overheadinformation in transmitted signals.

[0133] An alternative or adjunct to using a time requirement, as in step912, is to employ a minimum change in signal strength requirement in astep 914. In this mode of operation, the current pilot signal isrequired to decrease below a pre-selected second threshold before a newpilot signal is selected. That is, the pilot signal strength mustdecrease below some percentage of the value used to select that pilotbefore another pilot is used as the basis for switching to a new beam orsector service area. This requirement forces the user terminal to staywith a viable communication link longer and not switch beams until theuser terminal is clearly moving deeper into a new beam, and not simplyskirting the edge of a new beam in some more transient manner.

[0134] Using the above techniques, a user terminal can efficientlyselect pilot signals and move between beams being projected by a singlesatellite, without losing communications, yet using a minimum amount ofsystem resources. Likewise, a user terminal can effectively select pilotsignals while moving between adjacent sectors in a cell. Transmittedpower, which is one of two primary limitations on system user capacity,is minimized, for a given signal to noise ratio, on the ground forsatellites, if all of the power is diverted into beams with largersignal strength.

[0135] The previous description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. The various modifications to these embodiments will bereadily apparent to those skilled in the art, and the generic principlesdefined herein may be applied to other embodiments without the use ofthe inventive faculty. Thus, the present invention is not intended to belimited to the embodiments shown herein but is to be accorded the widestscope consistent with the principles and novel features disclosedherein.

What I claim is:
 1. A method for performing handoff between adjacentservice areas in a wireless communication system that transferscommunication signals using at least one central communications stationwhich establishes geographical service areas for user terminalsoperating within the system, comprising the steps of: detecting aphysical transition of a user terminal between a first service area anda second adjacent service area, each established by said one centralcommunications station, by detecting signal strength for signals fromsaid adjacent service areas; requesting a forward link channel for useby said user terminal in said second service area, while said userterminal also communicates using said first service area for forwardlink communications, when a detected signal strength for the secondadjacent service area at least equals that of said first service area;confirming that said forward link channel for said second service areais operating according to a preselected minimum quality level; anddisengaging use by said user terminal of the forward link for said firstservice area upon confirmation of said preselected minimum qualitylevel.
 2. The method of claim 1 further comprising the steps of: usingsaid central station as a gateway; and establishing said first andsecond adjacent service areas using first and second satellite beams,respectively, from a single satellite.
 3. The method of claim 1comprising the steps of: using said central station as a base station;and establishing said first and second adjacent service areas usingfirst and second sectors, respectively, from said single base station.4. The method of claim 1 wherein said step of detecting a transitionbetween first and second service areas comprises the step of detecting anew pilot signal associated with said second service area; and the stepof detecting signal strength comprises detecting a relative signalstrength of said new pilot signal.
 5. The method of claim 4 wherein eachservice area has a corresponding pilot signal, and said step ofdetecting a new pilot signal comprises the steps of: receiving pilotsignals with at least one user terminal receiver; measuring signalstrength of each received pilot signal; and comparing pilot signalstrength measurements.
 6. The method of claim 5 wherein said step ofcomparing pilot signal strength measurements further comprises the stepsof: detecting the presence of pilot signals adjusted to compensate forsignal roll-off conditions; and applying a compensation value to saidstrength measurements for each said adjusted pilot signal to obtain anon-adjusted value.
 7. The method of claim 4 wherein each service areahas a corresponding pilot signal, and said step of detecting a new pilotsignal comprises the steps of: receiving pilot signals with at least oneuser terminal receiver; measuring signal strength of each received pilotsignal; and reporting said signal strength of each received pilot signalto said central station.
 8. The method of claim 7 further comprising thesteps of: receiving said signal strength of each received pilot signalat said central station; and comparing pilot signal strengthmeasurements.
 9. The method of claim 8 further comprising the step oftransmitting at least one signal to said user terminal indicatingresults of said comparison step.
 10. The method of claim 7 furthercomprising reporting said signal strength of each received pilot signalto said central station on a predetermined periodic basis.
 11. Themethod of claim 7 wherein said step of receiving pilot signals furthercomprises the step of detecting the presence of pilot signals adjustedto compensate for signal roll-off conditions.
 12. The method of claim 1wherein the step of detecting a transition between first and at least asecond service areas comprises the step of detecting a new paging signalassociated with said second service area; and the step of detectingsignal strength comprises the step of detecting a relative signalstrength of said new paging signal.
 13. The method of claim 1 whereinthe step of confirming comprises the step of determining when the newchannel has sufficient energy to maintain a desired level ofcommunication service.
 14. The method of claim 1 wherein the step ofconfirming comprises the step of determining when the new channel has asufficiently low error rate to maintain a desired level of communicationservice.
 15. The method of claim 1 further comprising the step ofinspecting values for at least one pre-selected communication parameter,and prohibiting execution of said requesting step when a minimum changein value has not occurred for said parameter since a new forward linkchannel was previously requested for said user terminal.
 16. The methodof claim 15 wherein said step of inspecting comprises the step ofdetermining when a pre-selected minimum period of time has passed sincea new forward link channel was previously requested for said userterminal.
 17. The method of claim 15 wherein said step of inspectingcomprises the step of determining when a pre-selected minimum signallevel has been measured for a current service area signal beforeexecuting said requesting step.
 18. The method of claim 15 furthercomprising the steps of: storing identifying information in a memorylocation for each service area used, up to a predetermined maximumnumber of such service areas, and for a predetermined maximum length oftime; and comparing newly detected and stored identification informationfor service areas to determine if a same service area is being detectedagain within said length of time.
 19. The method of claim 1 furthercomprising the steps of: synchronizing the timing of communicationsignals and forward link channels for said user terminal at said centralstation through both said first and second service areas upon receivinga request for a forward link channel for use by said user terminal insaid second service area; and disengaging use of the forward link ofsaid first service area and commencing use of said forward link channelfor said second service area by said user terminal at substantially thesame time.
 20. Apparatus for performing handoff between adjacent serviceareas in a wireless communication system in which system users transfercommunication signals using at least one central communications stationwhich establishes geographical service areas for user terminalsoperating within the system, comprising: means for detecting a physicaltransition of a user terminal between a first service area and a secondadjacent service area, each established by said at least one centralcommunications station, by detecting signal strength for signals fromsaid adjacent service areas; means for requesting a forward link channelfor use by said user terminal in said second service area, while saiduser terminal also communicates using said first service area forforward link communications, said request being made when a detectedsignal strength for the second adjacent service area at least equalsthat of said first service area; and means for disengaging use by saiduser terminal of the forward link for said first service area when saidforward link channel for said second service area exceeds a preselectedminimum quality level.
 21. The handoff apparatus of claim 20 furthercomprising: a gateway operating as said central station; and a singlesatellite establishing said first and second adjacent service areasusing first and second satellite beams, respectively.
 22. The handoffapparatus of claim 20 further comprising: a base station operating assaid central station; and first and second sectors of a cellestablishing said first and second adjacent service areas, respectively,using said single base station.
 23. The handoff apparatus of claim 20wherein said means for detecting transitions between adjacent serviceareas detects a new pilot signal associated with said second servicearea.
 24. The handoff apparatus of claim 23 wherein said user terminalcomprises: at least one pilot signal receiver, for receiving pilotsignals each corresponding to a service area; means for measuring signalstrength of each received pilot signal; and comparison means forcomparing pilot signal strength measurements.
 25. The handoff apparatusof claim 24 further comprising: means for detecting the presence ofpilot signals adjusted to compensate for signal roll-off conditions; andmeans for applying a compensation value to said strength measurementsfor each said adjusted pilot signal to obtain a non-adjusted value. 26.The handoff apparatus of claim 23 wherein said user terminal, comprises:at least one pilot signal receiver, for receiving pilot signals eachcorresponding to a service area; means for measuring signal strength ofeach received pilot signal; and message transmission means for reportingsaid signal strength of each received pilot signal to said centralstation.
 27. The handoff apparatus of claim 26 wherein said reporting ofsaid signal strength of each received pilot signal occurs on apredetermined periodic basis.
 28. The handoff apparatus of claim 26further comprising: message reception means at said central station forreceiving said signal strength of each received pilot signal; andcomparison means for comparing pilot signal strength measurements. 29.The handoff apparatus of claim 28 further comprising user terminalmessage means for transmitting at least one signal to said user terminalindicating results of said comparison step.
 30. The handoff apparatus ofclaim 28 further comprising means for detecting the presence of pilotsignals adjusted to compensate for signal roll-off conditions.
 31. Thehandoff apparatus of claim 20 wherein said means for detectingtransitions between adjacent service areas detects a new paging signalassociated with said second service area.
 32. The handoff apparatus ofclaim 20 wherein said means for disengaging comprises means fordetermining when the new channel has sufficient energy to maintain adesired level of communication service.
 33. The handoff apparatus ofclaim 20 wherein said means for disengaging comprises means fordetermining when the new channel has a sufficiently low error rate tomaintain a desired level of communication service.
 34. The handoffapparatus of claim 20 further comprising hysteresis means for inspectingvalues for at least one pre-selected communication parameter, andprohibiting transfer of a new channel request when a minimum change invalue has not occurred for said parameter since a new forward linkchannel was previously requested for said user terminal.
 35. The handoffapparatus of claim 34 wherein said hysteresis means determines when apre-selected minimum period of time has passed since a new forward linkchannel was previously requested for said user terminal.
 36. The handoffapparatus of claim 34 wherein said hysteresis means determines when apre-selected minimum signal level has been measured for a currentservice area signal before requesting a forward link channel for saiduser terminal.
 37. The handoff apparatus of claim 34 further comprising:a memory in which identifying information for each service area used, upto a predetermined maximum number of such service areas, are stored fora predetermined maximum length of time; and means for comparing storedand newly detected identification information for service areas todetermine if a same service area is being detected again within saidlength of time.
 38. The handoff apparatus of claim 20 furthercomprising: timing means for synchronizing the timing of communicationsignals and forward link channels for said user terminal at said centralstation through both said first and second service areas upon receivinga request for a forward link channel for use by said user terminal insaid second service area; and control means for disengaging use of theforward link of said first service area and for commencing use of saidforward link channel for said second service area by said user terminalat substantially the same time.